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 Post subject: Re: A Dumb Question About Harmonics
PostPosted: Dec Sun 16, 2018 1:55 pm 
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OK---having no 20-Hz transformers, I'm still stuck on the theory. If I'm doing this right, the subharmonic generator works only on odd ratios---eg 20 is 1/3 of 60. In this case, when the lower frequency is at a zero-crossing, the higher frequency is also at a zero-crossing, AND the instantaneous slopes of each waveform are the same. This, the higher-frequency signal gives the required boost to sustain the oscillation at the lower frequency.

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 Post subject: Re: A Dumb Question About Harmonics
PostPosted: Dec Sun 16, 2018 6:31 pm 
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I imagine a waveform like this on the transformer:
Attachment:
60-20.jpg
60-20.jpg [ 19.18 KiB | Viewed 1020 times ]

The choke is designed to saturate on the highest peaks, which results in current pulses at a 20 Hz rate, maintaining the 20 Hz output. If the choke didn't saturate, the 20 Hz tuned circuit would just ring down to zero.

The circuit needs a push to start - the 20 Hz component won't be there at the beginning. The relay provides this kick - and the coil is probably sized so that it will drop if the resonant circuit isn't drawing enough current. And it will kick again until it starts.

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 Post subject: Re: A Dumb Question About Harmonics
PostPosted: Dec Mon 17, 2018 12:42 pm 
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So---it does appear that this works only the odd subharmonics. Do you imagine that it will work at 1/5 (12Hz)? 1/7?

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 Post subject: Re: A Dumb Question About Harmonics
PostPosted: Dec Fri 28, 2018 4:03 am 
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Its possible to generate frequencies lower than a given one
using only "passive" (sort of) components. This is called
parametric oscillation. However, it does require a bit of power.

It works with any nonlinear device, such as a diode. You feed RF in at
, say, 100 MHz. Attached are a diode and an LC circuit tuned to, let's say
35 MHz. You get out both 35 and 65 MHz! The 60 -> 20Hz circuit uses saturation
in a transformer.

For years I used this principle in my work. In went light at 1.064 microns,
into a crystal of lithium niobate 1x1x5 cm. If all went well,
I put in 10e8 watts/cm2 for 10e-8 second, and out came
out light at approximate 1.5 and 3 times that (2/3 and 1/3 the frequency) ,
tunable. If all did not go well, the crystal, which cost $4000, exploded.
This happened about once a year for 10 years. These days there are
somewhat cheaper crystals much less prone to explosion. This works because
at that power level the atoms in the crystal respond nonlinearly to the light.
They respond differently if the electric field points up compared to down.
Any piezoelectric crystal does this. Its just that most always explode
after one shot.

This light went into molecules. From them you got light at lots of lower
frequencies (not tunable ... depends on the molecule).


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 Post subject: Re: A Dumb Question About Harmonics
PostPosted: Jan Wed 02, 2019 11:48 pm 
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Imagine a motor driven crank turning at the input frequency, say 12 rpm (period is 5 seconds). Nearby we hang a pendulum which is tuned to 4 cycles per minute (period is 15 seconds). The pendulum bob is given a swing so it runs into the spinning crank handle, just a gentle bump will do. The crank kicks the pendulum away, but it will swing back in 15 seconds. Meanwhile the crank spins three times before it bumps the pendulum again. It is dividing by 3.

Make the pendulum longer so it swings slower at 20 seconds per cycle (3 cycles per minute). The every 5 seconds crank will be there again to tap the pendulum after each of its 20 second cycles. It is dividing by 4.

I say odd or even does not matter.

The pendulum is the resonator at the output sub-harmonic frequency.

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 Post subject: Re: A Dumb Question About Harmonics
PostPosted: Jan Thu 03, 2019 5:42 am 
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As the OP, I had no idea my original question would turn out to have all these posts. What I had in mind was using my cheapo Mercury signal generator that only goes down to 125KC could be used to align the 60kc IF of a Hammarlund HQ170 somehow. Of course i already knew the answer before I posted my dumb question.


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 Post subject: Re: A Dumb Question About Harmonics
PostPosted: Jan Fri 04, 2019 4:59 pm 
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Willitwork wrote:
Imagine a motor driven crank turning at the input frequency, say 12 rpm (period is 5 seconds). Nearby we hang a pendulum which is tuned to 4 cycles per minute (period is 15 seconds). The pendulum bob is given a swing so it runs into the spinning crank handle, just a gentle bump will do. The crank kicks the pendulum away, but it will swing back in 15 seconds. Meanwhile the crank spins three times before it bumps the pendulum again. It is dividing by 3.

Make the pendulum longer so it swings slower at 20 seconds per cycle (3 cycles per minute). The every 5 seconds crank will be there again to tap the pendulum after each of its 20 second cycles. It is dividing by 4.

I say odd or even does not matter.

The pendulum is the resonator at the output sub-harmonic frequency.

For the sake of argument, I wonder if we should consider that crank to be active device? If you try to get the same functionality in an electric circuit, the equivalent would be to disconnect the driving signal from the sub harmonic resonator for part of the cycle. That requires an active device.

From a different viewpoint, imagine that crank being attached to the pendulum. In that configuration, I don't think the pendulum will keep going.

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 Post subject: Re: A Dumb Question About Harmonics
PostPosted: Jan Sat 05, 2019 5:38 pm 
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'Active' device, or 'nonlinear', or something like that. And it may be the 'gap' rather than the crank itself. Kind of like a neon bulb that synchronizes a subharmonic oscillator to a powerline input.

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 Post subject: Re: A Dumb Question About Harmonics
PostPosted: Jan Sat 05, 2019 7:07 pm 
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I'm struggling with the definition of "active device".....in this context, it seems that we are talking about anything thst is not passive--where passive means "dumb". For example, just attaching the crank to the pendulum would be "dumb", and the pendulum would not continue to swing.
Another potential definition is that an ACTIVE device is anything that adds energy to a resonant system such that there are sustained oscillations.

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 Post subject: Re: A Dumb Question About Harmonics
PostPosted: Jan Wed 23, 2019 8:29 pm 
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dtvmmcdonald,

I'm trying to understand the mechanism by which your crystal oscillated. I spent several years doing X-Ray diffraction studies. Along the way, I discovered FE4O5 decades before its "discovery". Knowledge of it and the improvement to a product were closely held secrets to retain a competitive edge. That was long enough ago that the ASTM crystal diffraction database was on index cards. Later, I designed X-Ray generators that excited a target with as much as 125kV at 300mA.

Crystals have several oscillation modes, the modes and their susceptibility to oscillation being determined by the crystal structure. Usually, only one is excited. Does your method depend on stimulation oscillation in multiple modes simultaneously, meaning vibrations in more than one plane? If, so I'm surprised more didn't explode.

John


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 Post subject: Re: A Dumb Question About Harmonics
PostPosted: Jan Thu 24, 2019 1:42 pm 
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There's nothing magical about subharmonics; they are simply frequencies related to the fundamental by the ratio 1/n where n is an integer other than 1. Harmonics are frequencies n times the fundamental. It is also possible under certain circumstances to have interharmonics, where n is not an integer.

Production of harmonics and subharmonics in electronics generally requires that some part of the producing circuit be nonlinear, but it does not necessarily call for an active component such as a transistor or a tube. A good example are subharmonics in AC power systems which can cause considerable trouble. They sometimes result when some office building or factory gets sold a load of power factor correction capacitors by a fly-by-night energy company that doesn't do its homework. The capacitors make the power system resonant at a sub-harmonic of the AC frequency; everything is fine until a surge shock-excites the system under just the right load conditions, then all hell breaks loose.

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 Post subject: Re: A Dumb Question About Harmonics
PostPosted: Jan Thu 24, 2019 1:52 pm 
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Chris;
Curious to know your opinion of this:
pixellany wrote:
I'm struggling with the definition of "active device".....in this context, it seems that we are talking about anything thst is not passive--where passive means "dumb". For example, just attaching the crank to the pendulum would be "dumb", and the pendulum would not continue to swing.
Another potential definition is that an ACTIVE device is anything that adds energy to a resonant system such that there are sustained oscillations.


If, for example, you just run a transformer into saturation, you don't generate any subharmonics----but know I've forgotten the original explanation of why the ring-tone generator works.

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 Post subject: Re: A Dumb Question About Harmonics
PostPosted: Jan Fri 25, 2019 4:52 am 
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True that you cannot produce sub harmonics with a saturated transformer core alone. But if the transformer is coupled to something else that can circulate energy at a sub-harmonic frequency, it will stimulate that other device or network and sub-harmonics will be produced.

Magnetic telephone ring generators used this principle in an interesting way. Certain steel alloys have very sharp saturation curves which can be used to create inductors which are very nonlinear. If such an inductor is connected to a capacitor, it may be able to resonate at a different frequency than one would expect from classic LC tuned circuit theory. This is called ferroresonance. In order to get the transformer to ring at a lower frequency, a means of "kicking" it magnetically or electromagnetically into the saturated state is needed. So a ring generator would have the primary winding of a ferroresonant transformer connected to the AC line and the secondary tuned to the desired sub-harmonic output frequency with a capacitor. To ensure saturation, a starting diode or rectifier was typically connected in series with the primary winding. The rectifier had a resistor in parallel with it so a highly distorted sine wave input with a DC component would be applied to the primary.

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 Post subject: Re: A Dumb Question About Harmonics
PostPosted: Jan Fri 25, 2019 7:35 am 
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Pixellany,

I think your second definition is still holding up. The crystal gets a huge kick start. At the moment, I don't know if that is sustainable or its just another pendulum or more likely a ringing bell. It sounds like multiple modes are stimulated, which explains why there is an occasional explosion.

John


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 Post subject: Re: A Dumb Question About Harmonics
PostPosted: Jan Fri 25, 2019 12:45 pm 
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Quote:
As the OP, I had no idea my original question would turn out to have all these posts.

Having been here a little while now, I'm starting to detect a trend .... put into technical terms, "this is what happens." :). I think at least you got your original question answered this time.

This thread has turned into an interesting refresher course on harmonics of sorts. I don't deal with this stuff at all any more, so it's like getting an education all over again.

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